BRISBANE — A developer bulldozed an area containing endangered butterfly habitat on San Bruno Mountain last week, catching city officials off-guard and enraging environmentalists who plan to file a lawsuit to prevent the company from preparing the land for homes to be built.

Although the bulldozing appears to be legal, environmentalists with San Bruno Mountain Watch say the mechanized removal of vegetation on a portion of the Northeast Ridge owned by Brookfield Homes was a cynical attempt to wipe out the last vestiges of habitat for the endangered Callippe Silverspot and Mission Blue butterfly species.

Their concerns prompted a site inspection by Brisbane and county officials on Monday afternoon to make sure that the work was in line with what was approved by the county. The officials' conclusions were not available by press time.

The developer removed the vegetation early last week on part of a 20-acre site slated for 71 townhouses on a hill overlooking San Francisco.

A county consultant approved the work in mid-September, ostensibly for erosion and sediment control of the hillside above a roadway and existing development.

On Sept. 22, the San Mateo County Board of Supervisors issued a separate and unrelated permit that would allow construction to proceed starting early next year, pending final approval by the Brisbane City Council.

Last week's site work was, therefore, expected at some point, and Brookfield Homes prepared for it in August by uprooting 250 Johnny Jump-ups, the host plant of the Callippe Silverspot, and moving them to a safe place on another hillside.

It was the timing of the work that surprised Ken McIntire, executive director of San Bruno Mountain Watch.

He wonders why Brookfield Homes chose to remove native vegetation and replace it with special grass seeds to minimize erosion when the developer could have left the hillside alone until next spring, when grading will occur in earnest.

Leaving the property alone would have prevented erosion and given the butterflies a few more months to propagate, said McIntire.

"Why are they spending all this money to scrape away plant material that was preventing problems and spend more money to replace it with mechanical, artificial ways of doing the same thing?" he asked. "Next spring they'll be doing a heck of a lot more than scraping, so why do it twice? I think they wanted to get rid of the habitat as quickly as possible. Now Brookfield can say that there's no butterfly habitat on their building site any more."

San Bruno Mountain Watch founder David Schooley discovered the soil where the plants had been last Sunday while on a nature walk. The hillside near a sparse grove of eucalyptus is now bare but for some rocks and dirt. Workers have installed a "waddle" — a long flexible barrier to catch sediment — at the base of the cutting.

"I looked at the spot where the Silverspot and Mission Blue are — the place I'd been checking for years. It was sheared off. I was horrified," he said.

Schooley called Brisbane City Manager Clay Holstine, who was surprised at the news. He said the city had told the developer to "button up" the construction site by Oct. 15 — in other words, cover it for the winter — but he said he had no reason to believe Brookfield Homes was planning to touch the hillside zone known as a protected area.

"We would have said, `Why are you doing that now? You're not going to be doing any construction until next spring,' " said Holstine.

City and county officials seemed to disagree about who has final responsibility for approving work of this nature.

Holstine said the Brisbane Public Works Department did not sign off on the bulldozing work ahead of time because it's not part of the process.

"The city staff isn't out there overseeing where the work is going to be done — that's not their responsibility because that's what the county does."

San Mateo County Parks Planner Sam Herzberg said the county sent a consultant to the site to "fine-tune" the plan just before the vegetation was removed.

Herzberg said the review approval he issued was based on erosion control measures approved by Brisbane — the lead agency in such matters. Whether or not the work should have been done is beyond the county's purview, according to Herzberg.

"It's for the erosion control engineer and the city of Brisbane to determine. I didn't come up with this plan."